Compound transmissions of the range or combined range/splitter type are typically employed with heavy-duty vehicles, such as transport trucks. Transmissions of this type generally include a multiple speed main transmission section connected in series with a range type auxiliary section, wherein the range step of the auxiliary section is greater than the total ratio coverage of the main transmission section. The main transmission section may be shifted directly or remotely by a manual shift lever. The auxiliary section may be shifted by a slave actuator and is pneumatically, hydraulically, mechanically and/or electrically operated in response to manual operation of one or more switches. The range section may utilize synchronized jaw clutches. A range shift should be initiated and completed while the main transmission section is in neutral, to provide acceptable shift quality and to prevent undue wear and/or damage to the synchronized jaw clutches.
Compound range type transmissions typically include a control system that allows a range shift to be preselected using a selector button or switch at a master control valve. To prevent damage to the jaw clutches, the control system prohibits shift initiation until the main transmission section is shifted to, or at least towards, the neutral condition. One method for preventing shift initiation utilizes mechanical type interlock devices on the range section actuator mechanical linkage that physically prevents movement of the range section shift fork until the main section is shifted into neutral. A second system for preventing shift initiation utilizes logic-based interlock devices of the type wherein a valve supplying pressurized fluid to the range section cylinder is either disabled or not provided with pressurized fluid until a shift to main section neutral is sensed, or is only activated and provided with pressurized fluid while the main section is shifted to and remains in neutral. Examples of such transmissions and their control systems may be seen by reference to U.S. Pat. Nos. 2,654,268; 3,138,965, 4,060,005 and 4,974,474, the disclosures of which are hereby incorporated by reference.
Consistent and reliable operation of compound range type transmissions generally requires the ability to accurately detect the neutral condition of the transmission. Options for detecting the neutral condition of the transmission may include transmission shift bar housing neutral sensing devices comprising a plurality of separate sensing devices or switches for sensing axial displacement of each of a plurality of separate shift rails from an axially non-displaced neutral position. Illustrative examples of such mechanisms are disclosed in U.S. Pat. Nos. 4,722,237 and 4,445,393, which are hereby incorporated by reference. Other options include transmission shift bar housing neutral sensing devices comprising an axially displaceable rod having a plurality of specially contoured ramps and grooves (often referred to as a “Christmas tree” rod) arranged transverse to the longitudinal axes of the shift rails and cooperating with corresponding grooves/ramps formed in the shift rails. Illustrative examples of such mechanisms are disclosed in U.S. Pat. Nos. 4,676,115 and 4,290,515, which are hereby incorporated by reference.
The position of the transmission shift rails may not, in all instances, provide the most accurate indication of the actual neutral condition of the transmission. For example, certain compound range type transmissions, such as those including an overdrive gear range, may employ an X-control type mechanism to maintain a shift stick or shift lever pattern as a conventional progressive “H” type pattern for the forward speed ranges, including overdrive. The X-control mechanism operates to reverse the internal shift fork movement for the final speed changes to enable the overdrive speed range to be selected in the final position of the shift stick or lever. This is generally accomplished by employing a pivoted lever or “flipper” between the shift rail for actuating overdrive and the corresponding shift fork to reverse the action of the shift stick on the movement of the shift fork and cause the shift fork to engage the overdrive ratio in the final position of the shift stick. The pivoted lever or “flipper” causes the overdrive shift fork and the overdrive shift rail to move in opposite directions. Manufacturing tolerances and normally occurring wear of the X-control mechanism over the life of the transmission, however, may degrade the accuracy of using the position of the overdrive shift rail as an indication of the actual neutral condition of the overdrive shift fork, which may adversely effect operation of the range shift mechanism.